Bone is known as a dynamic organ which achieves bone reconstruction by constantly repeating formation and resorption for morphological change of the bone itself or for maintaining calcium concentration in the blood. In normal bone, osteogenesis by osteoblasts and bone resorption by osteoclasts have an equilibrium relationship, maintaining the bone mass in a constant state. However, when the equilibrium relationship between osteogenesis and bone resorption is disrupted, metabolic bone disorders such as osteoporosis are caused (Non-Patent Documents 1 and 2).
As bone metabolism-regulating factors, many kinds of systemic hormones or local cytokines have been reported and osteogenesis and bone maintenance are managed by interaction between these factors (Non-Patent Documents 1 and 3). The occurrence of osteoporosis is widely known as an age-related change in bone tissue. However, since the onset mechanism of osteoporosis involves many aspects including reduced secretion of sexual hormones or abnormality in the receptors therefor, changes in cytokine expression in local bone, expression of an aging genes, and differentiation or impaired function of osteoclasts or osteoblasts, etc., it is difficult to understand it as a simple physiological phenomenon which occurs with aging. Primary osteoporosis is mainly divided into post-menopausal osteoporosis due to reduced secretion of estrogen, and senile osteoporosis due to aging. For the elucidation of the onset mechanism and development of a therapeutic agent therefor, progress in basic research on regulatory mechanisms in bone resorption and osteogenesis is essential.
Osteoclasts are a multinuclear cells originating from hematopoietic stem cells, and by releasing chloride ions and hydrogen ions on their side adhered to bone they acidify the cleft between the cell and the adhesive side of the bone and simultaneously secretes cathepsin K, which is an acidic protease (Non-Patent Document 4). As a result, degradation of bone matrix protein and calcium phosphate is caused, yielding calcium recruitment into the blood.
The serum calcium concentration of healthy mammals is strictly maintained at about 9-10 mg/dl (about 2.5 mM) (i.e., calcium homeostasis). Parathyroid hormone (PTH) is a hormone which plays a key role in maintaining calcium homeostasis, and when the Ca2+ concentration in the blood decreases, PTH secretion from the parathyroid is immediately promoted. In bone, the PTH secreted accordingly recruits Ca2+ into the blood by promoting bone resorption, and in the kidney it promotes re-uptake of Ca2+ in the distal tubules, thus functioning of increase the Ca2+ concentration in the blood.
Because it is known that PTH can increase bone mass when it is intermittently administered to a human or an animal, it has already clinically been applied as a therapeutic agent for osteoporosis. Also, according to animal tests it has been reported that both osteogenesis and bone resorption of femoral cancellous bone are promoted by continuous administration of bovine PTH (1-84) to a rat from which the thyroid/parathyroid glands had been removed, consequently leading to an actual decrease in bone mass. However, subcutaneous intermittent administration thereof did not result in promotion of bone resorption but in promotion of osteogenesis alone, leading to an increase in the bone mass (Non-Patent Document 5). Furthermore, when human PTH (1-34) was intermittently administered to a rat for 15 weeks from 4 weeks post-ovariectomy, promotion of osteogenesis and inhibition of bone resorption were observed during the period from week 5 to week 10 after the start of the administration, showing an increased bone mass of about twice the bone mass of a sham operation group (Non-Patent Document 6). This report suggests that PTH not only prevents a decrease in bone mass in an osteoporosis model, but also has a bone mass recovery effect even in animals already suffering from a marked decrease in bone mass.
Although PTH preparations are therapeutic agents for osteoporosis which show a verified significant effect of lowering bone fracture rates according to clinical tests with patients suffering from post-menopausal osteoporosis, being biological preparations, they also have disadvantages. Specifically, injection has to be employed as the administration means, and therefore there is the problem that the patient may suffer pain associated with this. Thus, the development of a pharmaceutical preparation that can intermittently raise the PTH concentration in the blood and can be orally administered has been awaited.
The calcium receptor is a G protein coupled receptor which is mainly expressed in parathyroid cells, and it regulates PTH secretion by sensing Ca2+ concentration in the blood (Non-Patent Document 7). The human calcium receptor consists of 1,078 amino acids, and it is reported that the human calcium receptor is expressed in the kidneys, thyroid C cells, the brain, bone marrow cells, etc., as well as in the parathyroid gland. By binding Ca2+ as a ligand, the calcium receptor activates phospholipase C via coupling to G protein, causes the production of inositol triphosphate and an increase in the intracellular Ca2+ concentration and, as a result, suppresses the secretion of PTH (Non-Patent Document 8). Thus, it is expected that a pharmaceutical agent that inhibits activation of the calcium receptor, i.e., a pharmaceutical agent that antagonizes the calcium receptor, will promote PTH secretion from parathyroid gland cells and increase the PTH concentration in the blood of a living organism. In this regard, if the increase in blood PTH concentration is transient rather than continuous, it is expected to obtain the same bone mass-increasing effect as that provided by intermittent administration of PTH.
Meanwhile, as compounds containing an indanyl group, the compounds disclosed in Patent Document 1 are known. However, although they are similar to the compounds of the invention in that they include an indanyl group, the structure of the other end, etc. is different.